The present invention relates to videophones, and more specifically, to a method and system for videophones to initiate video communication while maintaining audio
Videophones are becoming popular because they permit visual communication without the need for traveling to a meeting place (by one or both parties of the communication). The saving in time and money in using videophones could be very significant. For a number of years, special telephone lines (such as ISDN) are used by videophones to meet the large bandwidth requirements of video communication. It is realized that videophones could be more useful if ordinary analog phone lines (called the "Plain Old Telephone Services", or POTS, lines) can be used for videophone communication. Recently, the International Telecommunication Union (ITU) promulgated a recommendation to support videophone over POTS and is entitled "Terminal for Low bitrate Multimedia Communication," designated as H.324.
Although POTS lines are the most widely available and least expensive telephone lines, there are disadvantages in using them for videophone communication. The most important disadvantages is probably the bandwidth limitation, currently at 33.6 kilobits per second (kbps). The effect of this limitation is that image quality and motion speed are less than desirable. A solution proposed by the present inventor, as described in PCT publication number WO 98/17053, is to use multiple telephone lines to increase the total bandwidth. As a result, the quality of videophone over POTS could be better than ISDN videophones.
Many conventional videophones require users to initiate a voice call first before a video call can be commenced. One reason for this design is for protecting the privacy of users. In these conventional videophones, audio communication is temporarily interrupted during the initiation of video communication. This is because videophones use modems, which need time to conduct digital handshaking and establish digital video communication. Normal audio telephone communication is temporarily suspended during these activities. It is found that many people feel uncomfortable when normal audio communication is lost in the middle of a telephone conversation. Consequently, it is desirable to maintain audio connection between the users during the whole phone call (i.e., both the audio-only period and video period).
Another problem with conventional videophone is that the modem handshaking may not be successful, and the telephone line may be dropped as a result. There may be a period of confusion after the line is dropped. The natural tendency of a user facing this situation is to redial the telephone number immediately. When both users redial at approximately the same time, the call cannot be established because the telephone system would indicate that the lines are busy. On the other hand, if both users wait for the other user to re-initiate a voice call the phone could be silence for a long time. This difficulty could be avoided if a voice link is always present at all times. In this case, the users can discuss on the phone the appropriate way to re-initiate the video link immediately after the link fails.